1. Field of the Invention
The present invention relates to an engine oxygen concentration sensor mounting structure in which an oxygen concentration sensor is mounted in an exhaust pipe connected to an outlet of a collector exhaust manifold of an engine.
2. Description of the Related Art
Conventionally known is a technique of detecting the oxygen concentration of an exhaust gas from an engine by an oxygen concentration sensor and controlling the amount of fuel injected into the engine based on the detected oxygen concentration. In a conventional multicylinder engine, an oxygen concentration sensor is generally provided in a collector exhaust pipe connected to the downstream side of a combined part of an exhaust manifold. In this case, it is known that equalizing the lengths of a plurality of exhaust single pipes forming the exhaust manifold can balance the contribution levels of the exhaust gases discharged from each of the combustion chambers, thereby enhancing the accuracy in detecting the oxygen concentration.
In a multicylinder engine in which the crankshaft is placed widthwise in the lateral direction in an engine compartment, it is comparatively easy to equalize the lengths of a plurality of exhaust single pipes. However, in a multicylinder engine in which the crankshaft is placed lengthwise in the longitudinal direction, an exhaust single pipe extending to a combustion chamber on the front side of the vehicle body inevitably becomes long, and an exhaust single pipe extending to a combustion chamber on the rear side of the vehicle body inevitably becomes short. As a result, when an oxygen concentration sensor is provided in a collector exhaust pipe connected to the downstream side of the combined part of an exhaust manifold, variations are produced in the levels contributed to the oxygen concentration by the exhaust gases discharged from the combustion chambers, leading to a problem of degradation of the accuracy in detection of the oxygen concentration.